Friction stir welding is a welding process exemplified in the accompanying FIG. 1, wherein a rotating friction stir welding tool 100—generally in the form of a cylindrical head 102 extending from a larger-diameter cylindrical shoulder 104—is rapidly rotated and contacted with a workpiece W to generate frictional heat and make the workpiece W plastic. The friction stir welding head 102 is plunged into the workpiece W, usually until its shoulder 104 abuts the surface of the workpiece W, to stir the area to be welded (generally a joint J between two pieces of material W1 and W2). The friction stir welding head 102 may be held in place to accomplish a spot weld, or it may instead be advanced along the joint J to form a linear weld, as by moving one or both of the friction stir welding head 102 and the workpiece W. In the case where the friction stir welding head 102 and the workpiece W are moved relative to each other, the head 102 is said to have an advancing side 106 (a side which moves in the same direction as the direction in which the head 102 moves relative to the workpiece W), and a retreating side (a side which moves in the opposite direction), which is not visible in FIG. 1. Once the weld is complete, the friction stir welding head 102 is removed from the workpiece W. Further details regarding friction stir welding can be found, for example, in U.S. Pat. No. 6,786,388 to Ezumi et al. (“Friction stir welding method”); U.S. Pat. No. 6,783,055 to Ezumi et al. (“Friction stir welding method and rotary tool”); U.S. Pat. No. 6,780,525 to Litwinski (“High strength friction stir welding”); U.S. Pat. No. 6,779,705 to Ezumi et al. (“Friction stir welding method”); U.S. Pat. No. 6,742,696 to Thompson (“Friction stir welding machine”); U.S. Pat. No. 6,722,555 to Ezumi et al. (“Friction stir welding method”); U.S. Pat. No. 6,715,664 to Yamashita (“Friction stir welding method”); U.S. Pat. No. 6,708,867 to Yoshinaga (“Friction stir welding method”); U.S. Pat. No. 6,676,004 to Trapp et al. (“Tool for friction stir welding”); U.S. Pat. No. 6,595,403 to Okamura et al. (“Friction stir welding method”); U.S. Pat. No. 6,554,175 to Thompson (“Friction stir welding machine and method”); U.S. Pat. No. 6,536,651 to Ezumi et al. (“Friction stir welding method”); U.S. Pat. No. 6,499,649 to Sayama et al. (“Friction stir welding apparatus”); U.S. Pat. No. 6,474,533 to Ezumi et al. (“Friction stir welding method”); U.S. Pat. No. 6,471,112 to Satou et al. (“Friction stir welding method”); U.S. Pat. No. 6,419,142 to Larson (“Apparatus for friction stir welding”); U.S. Pat. No. 6,367,681 to Waldron et al. (“Friction stir welding apparatus and method”); U.S. Pat. No. 6,360,937 to De Koning (“Friction stir welding”); U.S. Pat. No. 6,354,483 to Ezumi et al. (“Friction stir welding method”); U.S. Pat. No. 6,315,187 to Satou et al. (“Friction stir welding method”); U.S. Pat. No. 6,302,315 to Thompson (“Friction stir welding machine and method”); U.S. Pat. No. 6,299,050 to Okamura et al. (“Friction stir welding apparatus and method”); U.S. Pat. No. 6,237,829 to Aota et al. (“Friction stir welding apparatus”); U.S. Pat. No. 6,053,391 to Heideman et al. (“Friction stir welding tool”); U.S. Pat. No. 5,813,592 to Midling et al. (“Friction stir welding”); and U.S. Pat. No. 5,794,835 to Colligan et al. (“Friction stir welding”).
While friction stir welding is a valuable welding process, particularly for soft and/or thin workpieces, it tends to suffer from the disadvantages that the friction stir welding heads rapidly wear (particularly when harder materials are being welded). Additionally, it is often difficult to accomplish friction stir welding of materials with different melting points or other physical properties.